Location: Columbia River, OR
Mitchell Point was the site of an iconic tunnel with its five arched windows overlooking the Columbia River. Constructed in 1915, the tunnel was closed in 1953 because it could no longer accommodate increasing traffic volumes and larger vehicles. It was ultimately destroyed in 1966 to widen the water-level highway, now Interstate 84. The original tunnel was 390 feet long. Because Mitchell Point is a formidable mountain that separates key segments of the Historic Columbia River Highway State Trail, a new crossing was required to reconnect the western trail sections with Hood River and The Dalles.
The overall project consisted of pedestrian trail reconstruction, structures, stone masonry walls, draped rockfall protection fencing, tunnel excavation, traffic control, and related improvements along approximately 1.45 miles of trail between Cascade Locks and Hood River, Oregon. The new tunnel is approximately 647 feet long and was excavated in a horseshoe shape with a finished inside crown radius of 8.5 feet and vertical sidewalls 17 feet wide, providing a 12-foot-wide trail surface with 2-foot minimum shoulders. Beyond each portal, the tunnel transitions to a full-circumference shotcrete lining for its entire length. The tunnel also includes five viewing adits extending to the north, each constructed with a modified horseshoe-shaped profile slightly smaller than the main tunnel.
LRL’s scope included the installation of approximately 630 square yards of pre-stained draped rockfall protection fencing, including all anchors, wire rope supports, and associated hardware. Fabric panels were attached to top support ropes using ¼-inch lacing wire rope threaded through each mesh opening, with adjacent panels overlapped and securely fastened in accordance with project specifications and manufacturer recommendations.
Construction of the main tunnel and five viewing adits was completed using drill-and-blast methods. Approximately 8,000 cubic yards of rock were excavated during construction. LRL installed approximately 1,500 linear feet of #11 spiling bars around tunnel and adit openings, 12,000 linear feet of solid rock dowels throughout the crown and sidewalls, and fourteen W8x40 steel support sets along the western portion of the tunnel. The project also included the installation of approximately 5,000 square yards of structural shotcrete and 3,000 square yards of colored shotcrete within the tunnel and adits.
The work was completed safely, on schedule, and within budget.
